Not applicable.
1. The Field of the Invention
The present invention relates generally to surgical devices and methods for fusing adjacent bone structures and, more specifically, to surgical devices and methods for fusing adjacent vertebrae.
2. The Relevant Technology
The spinal column is made up of thirty-three vertebra each separated by a cushioning disc. Disease and trauma can damage these discs, creating instability that leads to loss of function and excruciating pain. Spinal fusion implants provide a successful surgical outcome by replacing the damaged disc and restoring the spacing between the vertebra, eliminating the instability and removing the pressure on neurological elements that cause pain. The fusion is accomplished by providing an implant which recreates the natural intervertebral spacing and which has an internal cavity with outwardly extending openings. The internal cavity is commonly filled with osteogenic substances, such as autogenous bone graft or bone allograft, to cause the rapid growth of a bony column through the openings of the implant.
Recently, adjustable fusion implants have been developed that allow the surgeon to adjust the height of the implant. This provides an ability to intra-operatively tailor the implant height to match the natural spacing between the vertebrae. This reduces the number of sizes that the hospital must keep on hand to match the variable anatomy of the patients. However, the prior art is replete with adjustable fusion implants that have an active mechanism for expanding the implant to change its height. Active mechanism refers to a mechanical structure built into the implant to cause the change in the height dimension. The presence of the active mechanism significantly decreases the amount of internal space available for placement of bone graft and other osteogenic substances to encourage the bony fusion between the adjacent vertebrae. It would therefore be an improvement over the prior art to provide an adjustable fusion implant that does not require the presence of an active mechanism, thereby maximizing the internal space for osteogenic substances and providing a better inducement for bony fusion.
Other adjustable fusion implants known in the art are comprised of modular components that must be pre-assembled prior to implantation. It would therefore be an advantage to provide a fusion implant that can be adjusted in situ.
Another challenge associated with spinal fusion is the restoration of the curvature of the spine. This curvature is present at each intervertebral level at varying degrees, and is manifested by a different spacing or height at the anterior and posterior margins of adjacent vertebral bodies. For example, the lumbar spine has a natural curvature when viewed from a lateral perspective referred to as lordosis, where the mid section of the lumbar spine is more anterior than the end sections. Thus, at any given intervertebral level, the intervertebral height at the posterior margin is less then the intervertebral height at the anterior margin, resulting in a wedge shaped disc or intervertebral space.
When a spinal fusion implant is placed from the posterior aspect of the vertebra, it must be sized to fit through the smaller posterior space, resulting in an undersized fit at the anterior end once the implant is in place. When the vertebral bodies are made to contact the opposing surfaces of the fusion implant, the curvature of the spine is straightened, producing higher stresses in adjacent levels of the spinal column and potentially leading to faster degeneration of adjacent intervertebral discs. Because some clinical problems require surgery from the posterior approach, it would be desirable to install an intervertebral fusion implant from the posterior side of the patient. It would therefore be an improvement to provide a spinal fusion implant that could recreate the natural curvature of the spine by reproducing the wedge shaped intervertebral space and concurrently allow for installation from the narrow side of the intervertebral space.
The present invention provides an adjustable bone fusion implant for selectively fusing together bones and/or pieces of bone. Methods are also disclosed for using and assembling the fusion implant. In one embodiment, the adjustable bone fusion implant comprises a first plate having an interior face and an opposing exterior face. Four spaced apart first support members project from the interior face of the first plate, each first support member having a rack of teeth projecting therefrom.
The fusion implant further comprises a second plate having an interior face and an opposing exterior face. The interior face of the first plate faces the interior face of the second plate such that a compartment is formed therebetween. Four second support members project from the interior face of the second plate. Each second support member has at least one tooth projecting therefrom.
The rack of teeth on each first support member mesh with the at least one tooth of a corresponding second support member. The meshed teeth enable selective separation of the first plate and the second plate but preclude unwanted collapsing between the plates. A plurality of grafting ports extend through each of the first and second plates so as to communicate with chamber. The grafting ports facilitate growth of bone through the fusion implant. A plurality of retention barbs outwardly project from the exterior face of the first and second plate. The retention barbs engage with the bone to be fused so as to help minimize migration or movement of the fusion implant.
Once the fusion implant has been expanded to fit a desired space, a reinforcing member can be inserted between the first plate and the second plate. The reinforcing member is positioned such that any compression load applied to the fusion implant is primarily carried through the reinforcing member as opposed to being carried between the meshed teeth. As a result, use of the reinforcing member substantially increases the amount of compression load that the fusion implant can bear prior to failure or permanent deformation.
In one embodiment, the fusion implant has a wedged shaped configuration so that it can be appropriate fit within a wedged shaped opening. For example, such wedged shaped fusion implants can be inserted between adjacent vertebrae.
In further accordance with the present invention, there is provided a method of installing the adjustable fusion implant. The components are first assembled in a fully collapsed state and connected to both an inserter and a distraction tool. The fusion implant is then placed between bones or bone parts to be fused. In the method discussed below, the fusion implant is inserted into an intervertebral space. The inserter is generally in the form of a solid rod. In one method, the fusion implant can be independently placed into the desired space by the inserter. The distraction tool can then be delivered to the fusion implant by referencing the inserter rod. In its collapsed state, the insertion profile of the fusion implant is less than the minimal spacing between the adjacent vertebrae. For lumbar spine applications, it is noted that the posterior spacing is less than the anterior spacing due to the spine curvature, or lordosis.
Next, the fusion implant is expanded by applying a distraction force from the distraction tool. The distraction force causes the meshed teeth on the support members to advance one tooth spacing at a time. Once the fusion implant is expanded to the size of the intervertebral space, the distraction tool is removed.
The next operative step is the introduction of the reinforcing member. The reinforcing member is aligned with or attached to a tubular push rod which in turn is advanced over the inserter. As the push rod is advanced, the reinforcing member is pushed into position between the first and second plates of the fusion implant. Once the reinforcing member is placed in its final assembled position, the inserter is removed. With the push rod still attached to the implant, the tubular push rod provides a channel in fluid communication with the chamber of the fusion implant. The channel can be use to deliver osteogenic substance, such as bone graft, to the compartment to facilitate bone growth. Once the osteogenic substance is delivered, the push rod can be removed. The completed operative technique provides restoration of the intervertebral spacing and restoration of the natural curvature of the spine through an approach from either the wide or narrow side of the intervertrebral spacing.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.